Multiple radio chains exist in almost all mobile radio communication systems, either per sector or per site. Normally, there is a unique power amplifier resource per transmission chain, and sometimes pooling of power amplifiers can be used to make an individual power amplifier available for several transmission chains.
Pooling of the power amplifier resource over a number of transmissions chains offers a number of potential advantages, such as graceful degradation and more efficient utilization of the resource, the latter being the focus of the present invention.
A pooled resource typically consists of three main blocks; an input matrix, power amplifiers (PA), and an output matrix. The matrixes are chosen such that the signal vector on the output ports of the output matrix is a power-amplified replica of the signal vector on the input ports of the input matrix.
The amplification is in general assumed to be equal for all of the power amplifiers in the pooled resource.
One frequently used realization of the input and output matrixes is the so-called Butler matrix. This matrix essentially performs a Discrete Fourier Transform (DFT). Other matrixes exist as well but these typically exhibit higher losses than a Butler matrix.
The (Butler) input matrix can be seen as a power divider which applies linear phase shifts over the output power divided signal vector. The magnitude of the phase shift depends on which input port is fed.
The power utilization of a pooled power amplifier resource depends on the characteristics of the signals to be amplified. If the signals are uncorrelated, each signal will be fed through all the power amplifiers with uniform power distribution over the PA-array. However, if the signals are correlated, or in fact identical except for a phase-shift and/or amplitude difference, the signals will not have a uniform power distribution over the PA-array.
The worst-case scenario, from a PA load-balancing scenario, is that all signals are fed through only one of the power amplifiers. In a steered beam system, the input signals are replicas of each other, except for a linear phase shift over the signal vector. For some steering angles, this phase shift causes the signals, after the input matrix, to flow via only one of the power amplifiers.
For most steering angles, signals are fed via all of the power amplifiers, however with a non-uniform power distribution. The power efficiency of the pooled amplifier is defined as the available output power for a given steering angle, when the power amplifier with the highest load delivers maximum output power, over the maximum output power per PA times the number of PA:s. The power efficiency will depend on the steering angle, but on the average is quite low.
If several independent signals are fed simultaneously via the pooled resource, as is the case with simultaneous users in the cell, the power efficiency will of course be higher due to averaging over user equipment locations.
An obvious solution to the problem of having non-uniform power distribution over the array is to place the power amplifiers in element space. However, in some applications it is desired to have the power amplifiers elsewhere than in element space, as will be discussed below.
One example of such an application is the case where the PA resource is used for transmission of either one data stream over a 4-element antenna array or two data streams over two 2-element arrays or four data streams over four single element antennas. As the power amplifiers are located in element space, i.e. at the antenna ports, the power efficiency becomes low, 0.5, for any data stream scenario since only half of the power amplifiers will be in use for a certain transmission scheme.
If the power amplifier resource is instead pooled with the aid of input/output matrixes, the power efficiency is improved to 1.0 for the transmission of 4 data streams, while it is reduced down to 0.25 for the single stream for the worst steering angles.